To prepare the marinade, combine the melted butter, soy sauce, vinegar, brown sugar, and garlic in a small bowl and whisk to combine. Pour the mixture over the cleaned mushroom caps in a large plastic bag and place in the refrigerator, turning every half hour until ready to grill.

Meanwhile, slice two ripe avocados lengthwise and mash roughly with a fork. Add lemon juice and salt (I used a seasoned salt mix from Penzey’s Spices, but you can really use any sort of salt for this purpose), stirring until the spread is malleable, but still slightly textured. Cover and set aside.

After about an hour of letting the mushrooms marinade, start slicing the vegetables into thick slabs. Preheat the oven to the lowest setting and remove the mushrooms from the marinade. Place them on a medium skillet over medium heat, tossing occasionally to avoid burning. Do not overcook. Place the cooked mushrooms on a tray with slices of bread and put in the oven to keep warm. Finally, sauté the onions and peppers until they begin to caramelize. Remove the mushrooms and bread from the oven. Spread both sides of the bread with avocado spread, place a mushroom, cap side down, on the bottom slice and fill the cavity with the vegetables. Top with the remaining slice of bread and enjoy.